The present invention concerns an apparatus and a method for detecting an object in a predetermined spatial region, in particular vehicles for traffic monitoring, according to the introductory part of the claims.
An apparatus of this kind is known from DE 42 34 880. The apparatus for detecting and recognising vehicles located on a roadway includes two narrowly focused distance sensors in order to be able to determine direction of travel, speed and vehicle length at the same time. By measuring the vehicle height which is determined from the measured, different distance values, classification of the vehicle model is possible. In this case the distance sensors are spaced apart in the direction of travel by a distance which is substantially shorter than the length of the vehicle. Also distance sensors have the advantage over ordinary reflex light barriers that higher reliability and better detection behaviour are achieved, as inaccuracies on account of distinguishing between beams which are reflected by a moving vehicle or by the road are reduced. To determine the speed, length and direction of travel of the vehicle, a pulse run time measurement is performed. A time measuring device for this purpose measures the time which elapses between detection of the vehicle by the first and second sensors.
From U.S. Pat. No. 5,321,490 is known an electronic object sensor for detecting objects which are in the vicinity of the sensor. Two focused, pulse-like laser beams diverging from each other are directed onto the area to be examined. The two beams are generated by means of a prism from a single laser beam emitted by a laser diode. The object sensor includes a receiver for detecting the beams reflected by an object in the area of observation. The run time which a pulse-like beam emitted by the transmitter needs until detection by the receiver is measured. The speed of an object in the area of observation is calculated from the distance which the two laser beams describe on the road surface, and the time which elapses between detection of the vehicle by the first beam and detection by the second beam.
From the reception of several successive pulses it is possible to deduce the number of vehicles, the vehicle size and shape, and hence the vehicle model.
A difficulty with the known systems however exists with respect to greatly fluctuating signal amplitudes which arise on account of the different reflection properties of the surfaces by which the light is reflected (e.g. road surface, plastic parts, windscreen or black metal parts). This means that surfaces the same distance away cause reception signals with essentially the same run time but different signal amplitude, which leads to difficulties in determining and fixing the moment of reception of the reflected signal. This causes in general uncertainty in time measurement of entry of the object into the laser beam, and therefore needs special precautions, for example an additional detector, in order to be able to correct this error.
It is the problem of the present invention, in detecting an object in a predetermined spatial region, in particular vehicles for traffic monitoring, to avoid the inaccuracies arising in case of run time measurements.
This problem is solved according to the invention.
A central concept consists in that detection of the object in a given region takes place by a comparison of an observed (measured) instantaneous value pattern with a previously determined instantaneous value pattern stored in a calibration table.
The advantages gained with the invention lie in particular in that a distance measurement can be determined by detecting a single backscattered or reflected radiation pulse without a timekeeper being needed. With the invention, measurement of the run time of a radiated pulse is completely avoided.
In traffic monitoring, traffic parameters such as for example number of vehicles, direction of travel and distance between the vehicles as well as vehicle speed, model, height and length can be detected. Also the invention advantageously allows detection of stationary vehicles within any selected time interval. By detecting several successive backscattered and reflected pulses it is possible to determine a profile of a vehicle. By a comparison with different patterns or characteristic features stored in a microprocessor unit, vehicle models can be recognised.
Another advantage of the invention lies in that the apparatus can be used in all weather situations, owing to the wavelength of the transmitter used. In addition the apparatus constitutes a component which is precise, reliable and cheap and requires only little maintenance.
By arranging three pairs of aligned laser diodes, of which the central pair is arranged in such a way that the radiation is directed perpendicularly relative to the area of observation, and the pairs to the left and right of the central pair are inclined by about xc2x112xc2x0, detection of the whole roadway can be achieved. In this case the apparatus is oriented vertically to the road surface. However, the apparatus can be mounted horizontally and mobile in vehicles. For the detection of traffic data of a multi-lane roadway, a plurality of sensor apparatuses can be run in parallel. Such an embodiment of the invention can similarly be used for traffic control.
However, the application of the principle according to the invention is not confined to the monitoring of traffic. Another application of the invention is for example the security monitoring of rooms.